In general what determines whether atoms will form chemical bonds

The number of electrons in its outermost shell.

Explanation:

Atoms react to form bonds when the number of electron in the outermost shell is not maximum i. e the shell can still accommodate electrons. A fluorine atom combines with itself to form a fluorine molecule because the number of electrons in its outermost shell is not complete (which is seven instead of the supposed eight). So, both atoms end up sharing an electron pair form a covalent bond.